The role of inducible nitric oxide synthase (iNOS) in the biochemistry and pathobiology of cardiac allograft rejection will be investigated. Previous work demonstrated that iNOS mRNA and enzyme activity was induced in macrophages infiltrating the myocardium and in cardiac myocytes during cardiac allograft rejection. The major hypothesis to be tested in this proposal is that NO in the rejecting graft augments myocardial inflammation and contributes to death of cardiac myocytes. These studies will address the following specific aims: 1) whether relatively selective iNOS inhibition will reduce myocardial inflammation and myocyte death in allografts, 2) whether inflammation and monocyte death is ameliorated by transplanting normal mouse hearts into mice genetically unable to induce iNOS (IRF-1 null mice), 3) if induction of cyclooxygenase-2 augments myocardial inflammation and myocyte necrosis, and if cyclooxygenase and iNOS inhibition ameliorate rejection. In addition, 4) a new strategy employing an arginine transport inhibitor which decreases NO synthesis will be examined for its effect on allograft rejection. Finally, 5) the mechanism by which NO produces death of cardiac myocytes will be investigated in detail. The results of the proposed experiments may provide new information concerning the role of iNOS in the pathobiology of cardiac rejection and suggest new therapies.